Recovery of petroleum by a cyclic thermal method



Sept. 3, 1968 BUXTON ET AL 3,399,722

RECOVERY OF PETROLEUM BY A CYCLIC THERMAL METHOD Filed May 24, 1967 A on. AIR on.

'8 l 3 I O 5 o THOMAS s. BUXTON JAMES E. PEASE INVENTORS ATTORNEY United States Patent 3,399,722 RECOVERY OF PETROLEUM BY A CYCLIC THERMAL METHOD Thomas S. Buxton and James E. Pease, Tulsa, Okla., as-

signors to Pan American Petroleum Corporation, Tulsa, Okla., a corporation of Delaware Filed May 24, 1967, Ser. No. 640,905

11 Claims. (Cl. 166-2) ABSTRACT OF THE DISCLOSURE An improved thermal recovery method is described in which a zone of high permeability is first created in a lower portion of the pay, after which ignition at that level is effected and combustion carried out for a period sufficient to heat the formation to a temperature ranging from about 700 F. to about 2,000 F. for a distance of at least 20 feet from the well; thereafter the well is killed by introducing a fluid into the lower portion of the pay, the well casing at the upper portion of the pay is perforated and oil of reduced viscosity is produced therefrom until the temperature of the oil approaches the original formation temperature, thereafter combustion in the aforesaid lower portion killing the well and subsequent production are repeated as described above. It is contemplated that an operation of this type can be carried out in a multi-well field with the process being employed in one or more of the wells simultaneously but at different stages.

Specification The present invention relates to the recovery of petroleum including heavy oils and tars from underground deposits thereof. More particularly, it is concerned with a novel procedure for recovering petroleum by thermal methods wherein the viscosity of the petroleum in place is substantially reduced through conduction of heat th-us rendering it less difficult to produce.

Background Large deposits of petroleum exist which cannot be produced by steam drive, miscible flooding, forward combustion or other secondary recovery methods. With the type of oil generally concerned in the process of our invention, i.e., having an API gravity not substantially greater than about 20, none of the aforesaid methods reduces the viscosity of the oil in the vicinity of the producing well bore until late in the life of the process. Moreover, processes such as steam injection are limited to depths of not more than about 3,000 feet, owing to excessive heat losses and higher injection pressures than cannot be avoided at greater depths.

In the case of steam injection procedures, a period of months is often required in order to heat up a sufficiently large body of the oil before the accompanying reduction in viscosity can be exploited. Thus for steam injection as well as the other processes mentioned above, the initial oil producing rate and hence the pay out of investment cost is directly related to the oil viscosity. As the oil viscosity increases, the ratio of producing wells to injection wells must also increase in order to maintain a satisfactory production rate. For example, in one commercial steam flooding operation in the Slocum Field, Texas, with which we are familiar, there are thirteen producing wells for one steam injection well.

In the now well-known huff-and-puif process for recovering petroleum in which steam is injected into a well for a period of time after which the steam saturated formation is allowed to soak for an additional interval prior to placing the well on production, much time elapses dur- 3,399,722 Patented Sept. 3, 1968 ing which no production is obtained. Also, the relative permeability of the formation to oil decreases owing to the increase in water saturation. One of the principal drawbacks of the huff-and-puff method is that it does not supply the reservoir with sufficient energy to move the oil toward the producing well in the quantities it should. When this process is used in reservoirs having little or no energy the oil moving to the producing well depends largely for its energy on that furnished by gravity drainage and by flash vaporization of hot condensate back in the formation when the well is depressurized and placed on production. Recent publications indicate that in the case of the common hutf-and-pulf procedure the economics are only marginal where steam is generated solely for this cyclic process. Hence, it is doubtful that huff-and-putf can be used economically unless operated in conjunction with a steam flood-which is limited to relatively shallow reservoirs and small (10 acres or less) well spacing.

Summary of invention We have now discovered that more efficient reservoir heating and deeper penetration of heat from the well bore can be accomplished by a heating method employing forward combustion in a special way. In accordance with a preferred embodiment of our invention, wells are drilled, for example, in a five-spot pattern, cased and completed by conventional methods except that a high temperature resistant cement is used opposite the pay zone. A suitable high temperature cement for this purpose is described in U.S. 3,180,748 and employs a combination of a high alumina cement and silica flour using a set retarder such as calcium lignin sulfonate or an alkali or alkaline earth metal phosphate. After the cement has set, a zone near the bottom of the pay is perforated. A horizontal barrier such as a shale stringer just above the perforations may separate the low narrower portion of the deposit from the upper, thicker section. If so, perforation at this level is usually all that is necessary to carry out the method of our invention. On the other hand, if the zone of interest is essentially homogeneous, a horizontal cut should be made in the casing or a planar charge used to form therein a series of perforations in a horizontal circular pattern. Thereafter, a horizontal fracture can be made within the reservoir extending well into it, e.g. -100 feet. In form ing such a fracture, a high flow capacity temperature resistant propping agent such as coarse sand, steel shot or glass beads, such as described and claimed in U.S. 3,155,- 162 may be used. The fracturing liquid employed is preferably one that will readily maintain the propping agent suspended therein during use, and which has a relatively low fiuid loss. Typical of such fracturing liquids are those described in U.S. 2.699.212.

As an alternative procedure, particularly where the petroleum containing deposit is substantially homogeneous, we may first form a combustion front in the lower portion of the deposit, as previously described, after which air or the equivalent is introduced into said deposit at said front at a pressure sufficient to lift the overburden or fracture the formation, thus creating a zone of high permeability and forcing the front to travel away from the well through which the air was introduced. We generally refer to this as the high pressure forward combustion step. In the case of tar sands, pressure is required to create such high permeability zones and may range from about 1200 to 1500 p.-s.i. at depths of the order of 1,000 feet. Such zone of permeability may also be created by the method just described prior to ignition.

The purpose of creating a fracture or a zone of higher permeability in the lower part of the pay is to establish a permeability contrast between the upper and the lower portions of the pay. In this connection it should be pointed out that the expression zone of high permeabilityas used herein is intended to. mean azone having a.

permeability of at least twice that in the upper portion of the pay. This permeability contrast will preferentially limit air movement to the lower portion ofthe pay. When a permeability. barrier such as a shale stringer or bedexists between the upper and lower portions of thepay, a high permeability zone may not need to be formed. for the barrier will restrict air movement to the lower portion. After the fracture has been placedor if as previously mentioned, a shale stringer divides the zone in which case a high permeability zone may not need to be formedthe formation is ignited thr-ough the perforations by means of any of several conventional ignition proce-.

dures, a preferred one of which is described and claimed in U.S. 3,223,165. By the use. .of such equipment the formation face can readily be heated to a. temperature of 1,000 F. to 1,500 R, if desired, after which injection of fuel for the ignitor is terminated. Air injection which is continued can be effected over a wide range-of conditions, however, usually it is conducted in a manner such as to introduce from about /2 to 2 million cubic feet of air per day. The combustion phase of our process should be r combustion front location, it.

At=injection time, days Z=oil consumed, 1b. oil/ft. of rock V= gas injection rate, s.c.f./dayft.

=gas density, lb./s.c.f.

N =oxygen concentration, lb. 0 lb. gas" w=oil oxygen ratio, lb. oil consumed/ lb. 0 injected The amount of heat added should be suificient to sustain long, e.g., 2 to 12 month periods of high oil production rates because of the lost production during the heating phase of the stimulation cycle and because of the cost of heating. The greater the heated radius, the longer the period of high producing rates. In terms of overall recovery per well, maximum oil recovery is obtained by the deepest penetration of heat into the reservoir.

Thereafter the introduction of air is discontinued and the well is killed by the introduction of water or drilling mud, etc., so as to prevent gases of combustion, air etc. from re-entering the Well while another set of perforations is made near the upper portion of the deposit. Next, the well is equipped with dual tubing strings as will be described in further detail below, the longer one of which extends to the combustion zone and the other is in communication with the last mentioned set of perforations to produce the oil flowing therefrom.

During installation of the tubing as Well as throughout the burning period, heat is conducted upwardly into the larger body of the reservoir. In the case of a homogeneous pay section any unreacted air from the burned zone tends to migrate to a higher structural position in the reservoir. In order for this air to move, oil must flow in the opposite direction. In doing so it contacts the air at a temperature, e.g. 200 F.40 0 F., suflicient for a spontaneous reaction. Thus the unreacted air in the burned out zone at the end of injection should normally be com"- pletely consumed during the shut in period which precedes the period of oil production. The time required to con' sume this unreacted air should be considered when deter-i mining the length of .the shutin period which typically, may be for 2 to 3 weeks. H

Production of hot oil of reduced viscosity may be continued until the temperature thereof falls to about the original temperature of said deposit; however, for economic reasons it might be desired to discontinue" production before such temperature isreached. Thereafter productionis discontinued; the well shut in andthe oil inthe productiontubing displaced back" 'intoithe formation (largely) Withwatepor other, suitable fluid to prevent the presence of appreciable. quantities of hydrocarbons in the well during the subsequent air injection step.

Air is next injected into the "lower set of perforations and the forwardcornbustion step repeated fora length of time preferably corresponding to the initial air injection periodfR'esidual heatcont'ained in the rock will eliminate themecessity of artificiaYignition as was required prior to thefir'st combustion phase. p i

' On completion ofthe'air injection step, the well is shut in to permit the heatt'o distribute itselfmore-evenly out awayfronfthe well and upward into the overlying oil zon'e i Preferably,theformation adjacent the producing perforations should be heated to a temperature of the order'of 200 F.2'50 F. This may require a shut in period of from several days to several weeks, e.g. 5 or days to 3' or 4 weeksl'After the shut in period, the well is placed on productidhQWhn the temperature of the produced oil approaches the original reservoir temperatore the previous three steps are repeated.

7 Description of drawings The method of our invention is further illustrated b reference to the accompanying drawings wherein i in wells 8, 12 and 14..

FIGURE 1 is a schematic representation of one embodiment thereof employing a five-spot pattern in which the wells shown are operating in accordance with various stages of our process;

FIGURE 2 represents a segment taken along line 2-2 of the pattern shown in FIGURE 1.

y In carrying out an'embodiment of our invention and referring to FIGURES 1 and 2, fractures 3 are obtained information 5 through perforations 4. The location of fractures 3 should be such that gravity forces will resaturate the burned out area with new fuel for the next combustion step. The formation is then ignited by means of'anyof several now well known ignition methods at a level substantially opposite perforation 4 in well 6v after which air-is injected into the heated zone via tubing 16 and continued forapproximately 2 to 25 days. During this time, tubing 20 is shut off thus permitting products of combustion to enter well 10 via perforations, 22 along with oil that was heated by previously carrying out the aforesaid ignition and combustion steps in well 10. These fluids are removed from the well through tubing 24. Some flow may occur into Well 10 below packer 26 and, if so,,can be withdrawn through tubing 20 which is not in use. Similar flow of fluids to varying extends may occur .Afterdiscontinuance of combustion by injection of air into fracture 3viaperforations 4 the well is killed by introducing water through tubing 16 into the confined space formedby packer 18 thus preventing fluids from backflowing into Wells 6. 1

In this connection it isto be pointed out that the process of our invention fundamentally is a production well stimulation technique and in practiceit is applied in a reservoir in-muchthe same fashion as cyclic huff-and-puif steam stimulationiscurrently used. All wells are not on the injection portion of the'cycle at the same time however. The wells on the production part of the cycle provide the drawdown which prevents undesirable buildup of're'servolrpressure; y p

=While'the' well is killed, perforations 30'are made in the casing after which hot'oil is produced into 'well 6 and up tubing 32'."Pro'duction may continue for several weeks or months, the duration of this phase of the process depending, usually, on the oil producing rate. When the oil temperature decreases to that of the original formation temperature, the production step should be discontinued and combustion by air injection resumed as described, after oil in production tubing 32 has been forced back into formation via perforations 30.

From the foregoing description, it will be appreciated that we have provided the art with an efiicient thermal recovery system that is free from any of the limitations characteristic of ordinary combustion and steam injection techniques. The process of our invention differs from the near well burn type of thermal stimulation in that all of the formation is not burned, the production interval is not the interval through which the stimulation has been carried out and the undesirable combustion products ordinarily are not produced with the oil. These factors make the process of our invention far superior to previously employed near well burn type processes since the problems encountered by the latter, such as production of corrosive and abrasive fluids, are eliminated. Our invention also has several advantages over reservoir heating by steam injection in that the heat is generated in the formation thus eliminating heat losses at the surface and at the well bore and a higher rate of heat transfer is obtained since the temperature difference normally is twice that obtained by steam injection. Vertical migration of air is not of any consequence since the production interval is shut in during the period of air injection, and hence the vertical flow of air is controlled by the normal pressure gradient in the reservoir.

We claim:

1. In a process for the recovery of petroleum from an underground deposit thereof, said deposit being penetrated by at least two wells, the improvement which comprises:

(a) igniting said deposit at a lower portion thereof at the face of the first of said wells,

(b) thereafter supplying an oxygen-containing gas to said ignited portion via the first of said wells to establish a combustion front and force the latter under suflicient pressure to move said front toward the other of said wells and continuing injection of said oxygen-containing gas until said lower portion is heated to a temperature ranging from about 700 F. to about 2000 F. for a distance of at least about feet from said first well,

(0) killing said first well by forcing a fluid into said lower portion, and

(d) producing petroleum of reduced viscosity from an upper level of said deposit and into said first well until the temperature of said petroleum reaches a value not less than the original formation temperature.

2. The method of claim 1 in which a zone of high permeability is established in the lower portion of said deposit and extends from said first well out into said deposit for a distance of at least about feet.

3. The method of claim 2 in which said zone of high permeability is established by means of hydraulic fracturing of said deposit.

4. The method of claim 2 wherein products of combustion 'and any unreacted oxygen are removed from said deposit via the other of said wells thereby preventing an excessive pressure buildup in said deposit which would interfere with fluid injectivity and result in possible fracturing beyond said deposit.

5. The method of claim 2 wherein more than two wells are employed and different stages of said method are simultaneously carried out in said wells.

6. The method of claim 2 wherein said petroleum deposit is a tar sand.

7. The method of claim 1 wherein said first well is shut in after the temperature of said petroleum decreases to about 150 F. to 200 F. and said petroleum in said first well is forced back into the upper level of said deposit, thereafter opening said first well and repeating step (-b), next shutting in said first well for a time sufficient to permit dissipation of heat through said deposit and the combustion products to migrate updip away from said first well.

8. The method of claim 1 wherein said first well is shut in after the temperature of the petroleum produced therefrom decreases to about 150 F. to 200 F. and the petroleum in said first well is forced back into the upper level of said deposit.

9. The method of claim 1 in which said deposit is divided by a horizontal substantially impermeable barrier.

10. The method of claim 1 in which said deposit is substantially homogeneous and the zone of high permeability is established in the lower portion of said deposit, said zone extending from the first of said wells out into said deposit for a distance of at least 75 to feet.

11. The method of claim 1 wherein said petroleum deposit is a tar sand.

References Cited UNITED STATES PATENTS 3,172,470 3/1965 :Huitt et al. 166-39 X 3,284,281 11/1966 Thomas 166-11 X 3,332,482 7/1967 Trantham 16611 X 3,333,637 8/1967 Prats 1662 X 3,342,258 9/1967 Prats 166-11 3,349,843 10/1967 Huitt 16611 X 3,349,847 10/ 1967 Smith et a1 16639 STEPHEN J. NOVOSAD, Primary Examiner. 

